mic-instrument-line levels 101

Mic level is the weakest. A microphone's output is tiny — we're talking millivolts. It's a fragile, low-amplitude signal that needs significant amplification before it's useful. This is what preamps exist to do — take mic level up to line level. The signal is also unbalanced in many cheaper implementations, making it susceptible to noise pickup over any meaningful cable length, which is why professional mics use balanced XLR connections.

Instrument level sits between mic and line, but it's a weird in-between that causes a lot of confusion. An electric guitar or bass puts out more voltage than a mic, but it's also very high impedance — typically 10kΩ to 1MΩ depending on the pickups. That high impedance means the signal is very sensitive to the input it connects to. Plug a guitar into a low-impedance input and the input loads down the pickups, sucking out high frequencies and killing the characteristic tone. This is why guitar amps and DI boxes have high-impedance instrument inputs — they're not just amplifying the signal, they're presenting the right impedance to let the pickup behave correctly.

Line level is the standard operating level for most audio gear — mixers, interfaces, effects processors, CD players, synthesizers. There are actually two standards: consumer line level at around −10dBV (used in home stereos, consumer gear) and professional line level at +4dBu (used in studios and live sound). These are about 12dB apart, which is roughly 4× the voltage. Plugging pro gear output into consumer gear input can clip it. Plugging consumer output into pro gear input means it arrives quieter than expected and you're amplifying more noise floor along with it.

Mic into a line input — the signal arrives way too quiet. You crank the gain to compensate, and now you're amplifying all the noise the cable and input stage picked up along the way. You get a usable signal buried in hiss.

Line into a mic input — the signal is massively too hot. It slams the preamp into hard clipping immediately. Sounds terrible, and depending on the preamp, sustained high levels can damage it.

Guitar into a line input — two problems at once. The level is wrong (instrument level is lower than line), and the impedance is wrong (line inputs are low impedance, which loads the pickups). You get a quiet, thin, tonally incorrect signal. This is why "just plug it into the interface's line input" doesn't work — you need the instrument/Hi-Z input.

Guitar into a mic input — impedance is still wrong, and now you're also dealing with a preamp designed for balanced low-impedance sources trying to handle an unbalanced high-impedance one. Noise, tone loss, and potential level issues all at once.

Synth or keyboard into an instrument input — a synth outputs true line level from a low-impedance output. Plugging it into a Hi-Z instrument input usually works okay but you're not getting the right impedance match, and on some interfaces it can sound slightly different than going into a proper line input.